It is known that one of the requirements for the obtaining of an acceptable quality of printing is the proper damping of the nonprinting parts of the plate (or matrix) before its final inking. This damping is effected by the feeding onto the matrix of a thin layer of water to which additives are added. The layer of damping liquid applied to the matrix must be continuous in order to avoid having the ink applied onto nonprinting parts of the matrix while at the same time it must not be too thick in order to avoid the damping liquid emulsifying to beyond the correct extent the ink which is used, with substantial detriment to the quality of the printing.
One of the most widespread damping systems, the so-called "traditional" system, uses, for the damping, water to which other substances are added, primarily phosphoric acid and gum arabic. It comprises a constant-level vessel into which a feed roller, also known as "fountain roller", dips which has a chrome-plated metal surface which at times is covered by a stocking. This "fountain" roller rotates intermittently with adjustable scraping width or slowly and continuously with adjustable speed so as to adapt the amount of water fed to the requirements of the job. Squeezing rollers or systems with concentrated jets of compressed air can gradate the dosage in transverse direction.
A so-called duct roller having an elastic covering covered by swanskin or "stocking" is journaled idle on its opposite ends on two oscillating arms. It takes the layer of water from the "fountain roll" and brings it into contact with a roller parallel to it, known as the "distributor", having a chrome-plated metal surface and imparted an axial reciprocating motion and a rotating motion, driven by gears at the same peripheral speed as the plate. The water fed to the "distributor" roller is transferred by the latter to two "damping" rollers with elastic covering and also covered with a swanskin or "stocking". They rotate idly, driven by friction with the plate and the "distributor", against which the contact pressure is registered by means of adjustable supports.
In this traditional system, the amount of water applied to the plate (or matrix) can be dosed very well. However, the system is not without substantial defects.
These defects are attributable in practically all cases to the fact that the system includes rollers covered with "stocking" or swanskin. The "stockings" are in fact capable of becoming dirty with ink and of transferring traces of the latter onto nonprinting zones of the plate or "matrix", with the consequent need of providing for their cleaning and at the same time for the replacement of the damping liquid, which also becomes contaminated.
It is furthermore known that, upon the starting of the machine, the system enters into equilibrium only after having imperfectly printed several sheets, with a resultant reduction in yield of the machine during each production cycle. At the end of each of the production cycles there is also indispensable a careful maintenance of all the rollers covered with "stocking" or swanskin, which consists in removing such rollers from the machine, washing their respective "stockings" and the rollers themselves, and then returning them into place in the machine. These are relatively lengthy and costly operations.
One substantial drawback resulting from the use of the "stockings" or swanskins resides in the necessity of replacing them after a certain period of use due to the wear which they undergo during the operation and during the washings carried out for maintenance. Replacement of the "stockings" is, as is well known, a difficult operation which requires a lengthy period of restarting before again reaching equilibrium operation of the machine.
In order to obviate many of these drawbacks of the so-called "traditional" damping, there has been devised the system of continuous damping in which the damping liquid consists primarily of water comprising the addition of alcohol which is adapted to lower the surface tension of the water, with the result that each drop, upon flattening out, wets a larger area of the plate or matrix, obtaining a proper water-ink equilibrium with the use of a smaller amount of water. Furthermore, the presence of alcohol in the damping liquid has the result that the liquid evaporates more rapidly, favoring the drying out of the fresh printing. The system is, however, functional provided that the liquid is distributed in a very thin uniform film without any discontinuity.
One example of a damping device developed with the above mentioned concepts is the one devised by DAHLGREEN. It also provides for the use of a constant-level vessel containing the damping liquid at a temperature and alcohol concentration which are also constant. The ordinary "fountain" roller with chrome-plated, smooth surface dips into said vessel, driven by an independent motor of adjustable speed and with a uniform peripheral velocity much slower than that of the plate. The "fountain roller" removes a certain amount of liquid which a "dosaging" roller, covered with hydrophilic elastic material and with a perfectly smooth surface, driven by the "fountain roller" at a peripheral speed not much different from the latter, calibrates in a layer which is as thin and uniform as possible. This layer is, in part, given up to the first of the inking rollers already charged with ink, which is driven by the plate so as to advance at the same peripheral speed as the latter. The differences in the peripheral speed on contact causes a spreading out of said liquid layer or film so as to make it even thinner. The shaft of the "dosaging" roller can be brought slightly out of parallelism with respect to the shaft of the "fountain" roller so as to increase the contact pressure in the central zone in order to compensate for the effects of flexure of the rollers. The thickness of the layer of damping liquid applied to the plate being thus calibrated in first approximation on basis of the printing requirements, the adjustments are made remotely, from the outlet of the sheets, varying the speed of rotation of the "fountain roller" by manual commands. This system has the advantage of eliminating the "stocking" or swanskin coverings and the drawbacks due to them, and of assuring the continuous relatively uniform and adjustable feeding, at least in theory, in very fine manner.
However, even with this system the regulating of the damping layer is practically unrelated to the speed of the machine. This fact creates problems during the operation of the machine, since the requirements upon start-up when the machine turns slowly are obviously different from those of a machine which has reached the normal operating speed.
The thicknesses of the film of damping liquid which can be applied to the plate (or matrix) which are obtainable with this system are not comparable to those which could have been created in the case of the "traditional" damping, in which the "stocking" retained a certain amount of liquid.
It was furthermore not contemplated or possible to obtain variation of the thickness of the liquid from zone to zone in transverse direction as might be necessary in view of the differences in the temperatures of the machine in the different zones or the variable characteristics from zone to zone of the plates.
Finally, it is known that also in this case all the adjustment operations for the purpose of maintaining the thickness of the film of damping liquid applied to the plate (or matrix) constant are effected with manual controls, on the basis of what is noted, more than any way else visually by the operator of the machine.